1. Field of the Invention
This invention relates to an electrophotographic developing device.
2. Background of the Invention
The developing operation of a conventional electrophotographic developing device will be described with reference to FIGS. 1, 2 and 3.
As shown in FIG. 1, a photosensitive drum 1 is rotated in the direction of the arrow (a) and is charged by a charging device (not shown). Under this condition, a latent image is formed on the surface of the photosensitive drum 1 by an optical device (not shown). Magnetic developing agent 3 is caused to adhere to the latent image by a developing device 2. That is, the latent image is developed into a visible image. The magnetic developing agent 3 is stocked in the developing device 2. A developing roll 4 is disposed beside the photosensitive roll 1 with a small gap therebetween. The developing roll 4 has been magnetized. Therefore, as the developing roll 4 turns in the direction of the arrow (b), the magnetic developing agent is transported in the direction of the arrow (c) so that it sticks on the photosensitive drum 1. In this connection, printing density depends on the quantity of magnetic developing agent 3 adhered to the photosensitive drum 1. One factor controlling the quantity of magnetic developing agent 3 stuck on the photosensitive drum 1 is the thickness of the layer of magnetic developing agent 3 conveyed by the developing roll 4. Therefore, in order to make the printing density uniform in the longitudinal direction of the photosensitive drum 1, a doctor board 5 is arranged along the photosensitive drum with a small gap therebetween. Since the gap between the doctor board 5 and the photosensitive drum is uniform, the layer of magnetic developing agent 3 conveyed by the developing roll 4 is made uniform in thickness, as a result of which the printing density is uniform.
As is apparent from the above description, in the conventional developing device, the edge of the doctor board 5 must be straight. That is, in FIG. 2, a front view of the developing device, the gap between the doctor board 5 and the developing roll 4 is uniform at any point; g.sub.1 =g.sub.2 =g.sub.3. However, as the developing roll 4 is rotated in the direction of the arrow (b), a load is formed in the gap between the doctor board 5 and the developing roll 4, so that the developing roll 4 is bent. That is, the gap between the doctor board 5 and the developing roll 4 is changed, with the result that the printing density is not uniform. If the layer of magnetic developing agent 3 on the developing roll 4 is excessively large in thickness, then the image developed on the photosensitive drum 1 is spoiled by the a magnetic stripe of magnetic developing agent 3, so that the printing quality is lowered.
The bend of the developing roll 4 will be described with reference to FIG. 3 in more detail. When a uniformly distributed load w is applied to the developing roll 4, then the developing roll 4, with bearings 6 at both ends acting as fulcrums, is bent as indicated by the broken line. The amount of deflection y of the developing roll 4 can be approximated according to the following quartic equation (1): EQU y=w(l.sup.3 x-2lx.sup.3 +x.sup.4)/24EI (1)
where x is the distance between either bearing 6 and a given point, E is the modulus of longitudinal elasticity, and I is the geometrical moment of inertia of the developing roll 4.
As is apparent from the equation, the amount of deflection is larger towards the middle of the roll 4 where g.sub.1 .perspectiveto.g.sub.3 &lt;g.sub.2.